1. Field of the Invention
This invention relates to a process for producing microcapsules for carbonless duplicating sheets comprising coating a hydrophobic core material with an aminoaldehyde polycondensate in a hydrophilic medium.
2. Prior Art
Recently, marked progress has been made in the techniques for producing microcapsules, and such techniques have been put into practical use for the preparation of perfumes, pharmaceuticals, adhesives, dyes, pigments, solvents and many others as well as carbonless duplicating sheets.
The techniques of producing microcapsules include mechanical processes, physicochemical processes, chemical processes and others. A technique proper for a specific purpose is used, but a coacervation process is frequently used because of its wide applicability. This coacervation process generally includes emulsifying a hydrophobic core material in gelatin, accumulating coacervation of a salt or a polyanion and gelatin around the emulsified droplets, forming capsule walls by cooling and curing/setting the capsule walls with formaldehyde or glutaraldehyde. This process has been widely used since the publication of U.S. Pat. No. 2,808,457.
However, in the application of coacervation process for the reduction of microcapsules for carbonless duplicating sheets, since gelatin is used as the principal material, the produced microcapsules are deficient in water resistance and in some cases the compactness of capsule walls is not sufficient. Above all, the process is complicated and requires delicate control.
In an attempt to solve these problems altogether, there have been proposed an interfacial polymerization process and an in situ polymerization process, comprising forming and accumulating synthetic resin films around cores of a hydrophobic material to form microcapsules.
For example, Japanese Patent Publication Nos. 446/1967, 2882/1967, 11344/1967 and 20069/1972 describe interfacial polymerization processes for producing polyurea wall capsules comprising polyurethane and a cyclic amine, epoxy resin wall capsules, polyester wall capsules and polyamide wall capsules. However, these processes have limited use because the kind of a core material to be encapsulated is limited owing to the strong reactivity of a starting monomer which forms wall films, and usually the capsule walls are thin and permeable.
On the other hand, the in situ process comprises supplying a reactive monomer or its precondensate from either one of a core material and an encapsulation medium and polycondensing or polyadding the monomer or the precondensate by heating or a like operation. As the wall materials used in this in situ polymerization process, there are mentioned almost all synthetic resins such acrylate resins, polyurethane resins, aminoaldehyde resins and polyester resins. Above all, aminoaldehyde resins have many advantages that the materials are easily available, the cost is low, no special catalyst is required for the resin forming reaction, the reaction temperature is low, i.e. under 90.degree. C. and the reaction time is short. Therefore, a number of studies have been made as described below.
Japanese Patent Publication Nos. 12380/1962, 12518/1963 and Laid Open No. 42380/1972 describe the use of melamine, urea, a polyhydric phenol, formaldehyde and a precondensate thereof as a starting material and the use of a surface active agent or gelatin as a dispersion stabilizer. However,m it has not been possible to obtain capsules having a good distribution and good compactness.
Japanese Patent Publication No. 30282/1971 and 23165/1972 describe that capsule walls are formed from a starting material which consists of urea/formaldehyde, methylated methylolurea/formaldehyde, dimethylated methylolurea/formaldehyde or a precondensate thereof. However, no dispersion emulsifier is used resulting in that capsules having a good distribution are not obtained. In addition the efficient accumulation of a capsule-forming material around the core material is not achieved; therefore, only thin capsule walls can be obtained.
Japanese Patent Laid Open Publication No. 144383/1976 describes a process in which capsule walls are formed by using a water-soluble polyhydroxy compound such as methylcellulose as an emulsifier and reacting this with an aldehyde or a precondensate of urea or melamine and formaldehyde. However, in this process, because the emulsifier itself takes part in the capsule-forming reaction, there is a difficulty in obtaining an efficient accumulation around the core material and therefore only thin wall films can be obtained.
Japanese Patent Laid-Open Nos. 84881/1978, 84882/1978 and 84883/1978 and Japanese Patent Publication No. 16949/1979 teach that it is possible to obtain capsules which are single particles, less agglomerated, tough enough to handle and good in film compactness, within a short time in the form of a capsule slurry in high yields and concentrations by employing, in a monomeric or precondensate form, urea or melamine and formaldehyde, methylated dimethylolurea, methylated methylolurea and methylolmelamine or methylated methylolmelamine as a starting material for capsule wall films and employing polyacrylic acid or bipolymer of maleic anhydride as a system modifier. Although the system modifier disclosed therein has some advantages of its own, it has drawbacks that the uniformity of a capsule size is somewhat poor, the viscosity of capsule slurry is yet high, the bipolymer of maleic anhydride requires much time for its dissolution, or the like. In order to overcome these drawbacks, a combination of a styrene/maleic anhydride copolymer and a vinyl acetate/maleic anhydride copolymer is used as a system modifier (Japanese Patent Laid-Open Nos. 49984/1979 and 47139/1980). This process can provide capsule slurry with higher solid concentration and lower viscosity. However, the process is not applicable to a urea/formaldehyde system or the like which requires wall forming conditions including a pH.ltoreq.4, because, when the pH in the reaction is not higher than 4, the styrene/maleic anhydride copolymer deposits.
Japanese Patent Laid Open Publication No. 51431/1980 describes that, in case where a polymer of formaldehyde and melamine is used as a capsule wall-forming material, a slurry with an extremely low viscosity can be obtained by adding gum arabic to the system. However, the stability of the emulsified particles is poor during the capsule wall-forming reaction, and only capsules with an extremely poor particle size distribution can be obtained.
Moreover, Japanese Patent Laid-Open No. 51238/1981 proposes that, in a process for producing capsules comprising using a melamine/formaldehyde polymer as a wall-forming material, a styrenesulfonic acid polymer is added as an anionic polyelectrolyte. However, this process has a difficulty in operability, for example, foaming during the dissolution of polymer, and the compactness of the obtained capsule walls is not satisfactory. Especially, when this process is applied to a system in which a urea/formaldehyde resin is used as a wall-forming material, a slight change in condition causes the entire system to agglomerate.
Japanese Patent Laid-Open No. 92135/1980 describes that, in a process for producing capsules comprising using an aminoaldehyde resin as a wall-forming material, an anion-modified polyvinyl alcohol, particularly a polyvinyl alcohol having carboxyl groups as the anions is used. However, when a high-saponification degree polyvinyl alcohol is used, in case where a core material is emulsified, the distribution of the emulsified particles is not good, the stability of the emulsified particles is not sufficient during the capsule forming reaction and only capsules with a wide particle size distribution are produced. A low-saponification degree polyvinyl alcohol causes an increase in slurry viscosity during the capsule forming reaction.
Japanese Patent Laid-Open No. 132631/1980 describes that in a process for producing microcapsules comprising using a urea/formaldehyde copolymer as capsule walls, microcapsules having good compactness can be obtained by using a polyvinyl alcohol having a saponification degree of at least 95% alone or in combination with a variety of water-soluble polymers. However, in this process, the viscosity increases during the capsule wall-forming reaction, resulting in agglomeration of capsules or a maldistribution of emulsified particles.
Furthermore, Japanese Patent Laid-Open No. 58536/1981 describes that, in a process for producing microcapsules comprising using a melamine/formaldehyde precondensate as a wall material, a sulfonic acid group-containing polymer or copolymer is added. However, the monomers used are uncommon in a process for producing microcapsules for carbonless duplicating sheets and, therefore, can not be widely used from the viewpoint of a process for polymerization, quality, cost, etc.